AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
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Observing, Predicting and Controlling Crystalline and Amorphous Particle Formation in Evaporating Aqueous Aerosol Droplets
JONATHAN P. REID, Florence Gregson, Rachael E.H. Miles, Jim Walker, Daniel Hardy, Justice Archer, Joshua Robinson, Royall Patrick, University of Bristol
Abstract Number: 412 Working Group: Nanoparticles and Materials Synthesis
Abstract The evaporation of aqueous aerosol droplets to form dry crystalline or amorphous particles is a fundamental process that is important for understanding the phase of atmospheric particles and is exploited in industrial processes such as spray drying. The phase, composition and heterogeneity of the final particle is dependent on the drying conditions (for example, relative humidity and temperature) and the presence of additives (for example, the presence of surfactants and co-solvents). Further, as a particle losses solvent and decreases in size, the viscosity and diffusivity of the different chemical components can vary over many orders of magnitude, slowing the loss of water and leading to amorphous particles that release moisture over very long-time frames. Using experimental tools for probing the rapid drying dynamics of single particles, new insights can be gained into the microphysical processes that determine the phase and microstructure of the final particle. Specifically, we will report measurements using an electrodynamic balance and using a falling droplet instrument, providing access to time-resolutions spanning from 10 μs to hours and allowing direct measurements of the evaporation kinetics, transient morphologies from high time-resolution imaging, and final particle morphologies from scanning electron microscopy. From these experiments, we will contrast the well-defined crystallization time and final morphologies of the binary salt sodium chloride with the behavior of alkali metal nitrates solution droplets, which often form amorphous particles. By incorporating long-chain alcohols, we will demonstrate control over the crystallization time and final particle size during droplet drying. We will also explore the different stages of drying of aqueous saccharide solution droplets, contrasting the rapid initial loss of water with the much longer stage of drying as the particle becomes increasingly viscous, reporting measurements of the evolving mass over many hours.